Method of multiple transfer of latent electrostatic images

ABSTRACT

AN ELECTROMAGNETOGRAPHIC METHOD OF PRINTING CONPRISES GENERATING A LATENT ELECTROSTATIC OR MAGNETIC IMAGE ON AN IMAGE CARRIER WHICH IS NOT MAGNETIZABE BUT IS ELECTRICALLY CONDUCTIVE. A MAGNETIZABLE, ELECTRICALLY CONDUCTIVE POWDER IS APPLIED TO DEVELOP THE IMAGE AND THE DEVELOPED IMAGE IS FIXED ON THE CARRIER BY AID OF A SUBSTANTIALLY UNIFORM MAGNETIC FIELD, WHICH HOLDS THE MAGNETIZABLE PARTICLES IN PLACE. WHILE SO HELD IN PLACE, THE IMAGE PARTICLES ARE CHARGED AND BROUGHT INTO CONTACT WITH AN IMAGE RECEIVING MATERIAL. THE CARGE ON THE MAGNETIZED PARTICLES INDUCES A CHARGE ON THE IMAGE-RECEIVING MATERIAL CORRESPONDING TO THE IMAGE. THE CHARGED IMAGE-RECEIVING MATERIAL IS THEN REMOVED AND THE IMAGE DEVELOPED AND FIXED IN THE KNOWN MANNER AS BY USE OF A TRIBOELECTRIC TONER. THE MAGNETICALLY-FIXED MASTER IMAGE MAY BE RECHARGED AND USED REPEATEDLY FOR GENERATION OF ELECTROSTATIC IMAGE CHARGES ON IMAGE CARRIER MATERIAL; WHEN THE REQUIRED NUMBER OF IMAGES HAVE BEEN PORODUCED, THE MASTER IMAGE IS REMOVED BY REMOVING THE MAGNETIC FIELD AND BRUSHING OR BLOWING AWAY THE PARTICLES.

United States Patmt Ci'f'fice 3,704,120. Patented Nov. 28, 1972 3,704,120 METHOD OF MULTIPLE TRANSFER OF LATENT ELECTROSTATIC IMAGES Horst Kosche, 14 Holzstrasse, 516 Duren, Germany No Drawing. Filed Jan. 14, 1971, Ser. No. 106,607 Int. Cl. G03g 13/18, 13/22 US. Cl. 961 R 21 Claims ABSTRACT OF THE DISCLOSURE An electromagnetographic method of printing corn,- prises generating a latent electrostatic or magnetic image on an image carrier which is not magnetizable but is electrically conductive. A magnetizable, electrically conductive powder is applied to develop the image and the developed image is fixed on the carrier by aid of a substantially uniform magnetic field, which holds the magnetizable particles in place. While so held in place, the image particles are charged and brought into contact with an image receiving material. The charge on the magnetized particles induces a charge on the image-receiving material corresponding to the image. The charged image-receiving material is then removed and the image developed and fixed in the known manner as by use of a triboelectric toner. The magnetically-fixed master image may be recharged and used repeatedly for generation of electrostatic image charges on image carrier material; when the required number of images have been produced, the master image is removed by removing the magnetic field and brushing or blowing away the particles.

This application pertains to the art of electrographic printing, and more particularly, to the printing of multiple copies of an original electrostatic or magnetic image.

In ferrographic or magnetographic printing processes latent magnetic images corresponding to written, printed or drawn information are formed on thin plates or layers of magnetizable material and the images are rendered visible by covering the plates or layers with a fine magnetic toner powder. The powder particles forming the latent images are then transferred to a printing or imagereceiving material, such as paper.

In electrographic or xerographic printing processes, latent electrostatic images corresponding to written, printed or drawn information are formed on a thin electrically non-conductive layer of photosensitive material and rendered visible by the application of a finely divided, triboelectrically charged electroscopic toner. The electrostatic images thus produced may be transferred to a printing surface by well-known methods and are then fixed to provide a permanent reproduction.

It is known (see, for example, US. Pat. 3,142,561, issued July 28, 1964) that in the generation of electrophotographic images the toner powder used for developing the latent image can be retained on its base electrostatically or magnetically.

It has also been proposed, for example in Canadian Pat. 607,291, issued Oct. 25, 1960, to transfer latent undeveloped electrostatic images from an imaging surface to a second surface of insulating material and to develop and fix the transferred electrostatic images.

The prior art methods appear to be deficient in several respects. For example, in transferring images by a transfer of toner particles, only a portion of the toner particles are transferred while some are left behind, and consequently the transferred image is not only lighter than the original, but only a limited number of transferred images can be made before the toner particles are exhausted.

Further, when an electrostatic image is transferred to an image carrier by bringing the carrier into close contact with the sheet carrying the electrostatic image, there is a tendency to displace the toner particles by the mechanical pressure of the image sheet.

The present invention contemplates a method of electrographic printing which provides for printing any number of copies of equal intensity from a single electrostatic or magnetic image by transferring an electrostatic master image any number of times to an image receiving surface in contact with the master image without disturbing the master image.

In accordance with the present invention, there is provided an electromagnetographic process comprising generating a latent electrostatic or magnetic image on an image carrier, developing the image with a conductive toner comprising electrically conductive, magnetizable particles, fixing the conductive toner image on the image carrier with the aid of a magnetic field, imposing an electric charge on the fixed conductive toner image, transferring the imposed charge on the image to an electrically insulating image receiving material and developing and fixing the transferred charge image. The foregoing steps may be repeated as often as required so that any number of images can be transferred and fixed. The magnetic field is extinguished and the conductive toner particles are removed from the surface of the image carrier when the required number of copies are made.

Electrostatic processes in general comprises the generation of a latent electrostatic or magnetic image on a carrier, which image is developed by a toner usually comprising a fine powder which clings to the charged area of the imaged surface and may then be fixed either on an image-receiving material to which it has been transferred, or in place, by suitable means such as heating, or solvent fixation.

In accordance with one aspect of the invention, an electrostatic or magnetic image is produced on a suitable surface supported by a base which is magnetically neutral but electrically conductive. A latent electrostatic image is produced on the surface and, in accordance with the invention, an electrically conductive magnetizing toner is applied which will cling to the charged portions of the surface to form a master image in magnetizable, electrically conductive toner particles.

Prior to bringing an image-receiving surface into contact with the image, a uniform magnetic field is imposed on the master image, which field holds the magnetizable particles in place so they are not mechanically displaced from their position by being brought into contact with an image-receiving material.

An image receiving material is then brought into contact with the image and an electric charge is applied to the master image particles, which induces a charge on the image-receiver surface. This induced charge creates on the image-receiving surface an electrostatic image corresponding to the master image. The image-receiving material is then removed. The magnetic field is maintained while the electric charge is imposed.

The steps of inducing a charge on the image-receiving material may be repeated as often as required, thereby transferring the electrostatic master image any requisite number of times.

In accordance with one aspect of the invention, an electrically conductive, magnetizable conductive toner material consisting of powders of iron, cobalt, nickel or alloys thereof is employed.

In accordance with another aspect of the invention, the electrical charge applied to the master image to transfer the electrostatic image may be imposed by using an electrically conducting brush to contact the image, ion radiation, generation of frictional electricity by suitable means brought into contact with the image, or a corona discharge.

It is an object of the invention to provide an electrographic process wherein unlimited numbers of electrostatic images may be transferred to image-receiving material which images are of uniform strength and clarity.

Other advantages and objects of the invention will become apparent upon consideration of the following detailed disclosure of specific embodiments of the invention.

An electrostatic image is produced on a suitable image carrier surface such as an insulating photoconductive layer, for example, an organic photoconductor in the form of polymers or resins in combination with resins. Suitable photoconductors for xerography are certain metal oxides and sulfides such as zinc oxide, certain metals such as selenium and selenium mixed with other elements such as tellurium, and certain organic compounds. Such materials may be embedded in an insulating resin matrix to form a thin film of photoconductive material.

The photoconductive material is supported on a base which is electrically conducting but non-magnetizable. A suitable base usable in accordance with the invention may be of aluminum, a suificiently conductive paper, a plastic material with a conductive coating, or some other non-magnetizable material with a conductive coating.

A latent electrographic image is formed on the image carrier, for example, by known methods comprising uniformly charging the photoconductive or other image layer and exposing the charged surface to an image. Light striking the photoconductive surface will cause the charge on the lighted portions of the surface to decay and the result will be an electrostatic image corresponding to the exposed image either in positive or negative form, depending on Whether the image or a reversed image of the original is exposed to the photoconductive layer.

The initial uniform charge may be applied by any known means such as by an electron beam or by electromagnetic radiation, or by applying a concentrated field of ions obtained from a corona discharge.

The latent electrostatic or magnetic image formed is developed by applying thereto a finely divided conductive toner comprising particles which are electrically conducting and magnetizable. Electric conductivity of the toner on the order of magnitude of metallic conductors is desirable. Accordingly, suitable materials for use as a conductive toner of the invention are fine powders of iron, cobalt, nickel or alloys thereof. These materials are preferred because of their availability, electrical conductivity and magnetizable properties. However, any metal or alloy with corresponding electrical and magnetizing properties would be suitable.

The conductive toner may be in the form of spheres or other shape particles made of metals as aforesaid, or other material with a coating of the conductive, magnetizable metal. The particles may also comprise conductively coated plastic or other material envelopes or coat ings over a core of magnetizable metal, and any mixture or combination of the foregoing.

The electrically conductive, magnetizable toner will cling to the charged portions of the image layer thereby developing the electrostatic image.

A substantially uniform magnetic field is then imposed to hold the developed image on the image carrier surface. This may be accomplished by placing below the base supporting the image carrier surface an arrangement of electromagnets, a magnetic tape or other suitable means for imposing a magnetic field to retain the conductive toner layer on the material. The magnetic field must be applied while the conductive toner particles are still held by the electrostatic forces forming the image. That is, the magnetic field may be applied any time prior to significant decay of the electrostatic or magnetic charge forming the image on the image carrier material. The magnetic field imposed renders the conductive toner image mechanically resistant, permitting the image to be submitted to the subsequent operations repeatedly bringing it into contact with image-receiving materials without distortion of the image as by loss or displacement of conductive toner particles.

While the magnetic field is maintained, the conductive toner particles are charged with an electric charge until a suitably high voltage is built up thereon. Preferably a voltage of between about 2 to about 20 kilovolts is developed. This can be accomplished as aforesaid by any known means, such as an electrically conducting brush, ion radiation, frictional electricity or corona ion discharge.

it is possible that the imposition of this high electrical charge will charge not only the conductor toner particles but also the particle-free surface area of the image layer. It has been found in practice that even though some charge may accumulate on the particle-free surface of the image layer, they do not interfere with clean transfer of the charge image to a receiving material. However, if it is necessary or desired to remove such charge this can be done by known means. For example, in a case of a photoconductive layer the image may be exposed to a uniform light exposure which will cause the charge on the surface area to decay.

The electrostatic image formed by imposition of the electrical charge on the conductive toner particles is transferred to an image receiving material. This is accomplished by contacting the conductive toner image with either a stationary or traveling image receiving material. The image receiving material preferably comprises a sheet base material having electrical conductivity on the order of 10- mhos per centimeter, and an electrically insulating layer that will accept a charge and that is not itself photosensitive.

The charge imposed on the conductive toner particles induces a charge on the image-receiving material corresponding to the image formed by the conductive toner particles. During the transfer or inducton of charge the magnetic field for fixing the conductive toner image is maintained thereby holding the particles in place during the transfer operation. The charge is induced in an imagereceiving material substantially only where the image carrier surface is covered with conductive toner. The conductive toner image thereby serves to transfer the charge in a manner analogous to that in which a typeface or screen transfers the ink in a relief printing operation. The image receiving material accepts a part at least of the charge imposed on the conductive toner image by induction in the insulating layer next to the image. After receiving the charge the image receiving material is removed.

Any suitable receiving material may be employed, for example, a plastic coated paper.

While any suitable physical form of image carrier surface and image-receiving material such as flat plates and sheets may be employed, it is convenient to have the image carrier surface and its support base in the form of a cylinder such as a conventional printing cylinder,

and to supply the image-receiving material from a roll of the same.

The electrostatic image thus transferred may be developed in a conventional manner with a triboelectric toner, and fixed by known means such as heating or solvent fixation.

The master image is still maintained by the magnetic field holding the conductive toner particles in place, and may now be recharged and repeatedly used for the generation of additional electrostatic images. The process may be repeated any required number of times since the continuously imposed magnetic field holds the particles in place notwithstanding the mechanical contact with the image receiving material. When a requisite number of electrostatic images and corresponding copies have been transferred and made, the primary image may be removed by simply switching off the magnetic field. The magnetizable conductive toner particles are then removed from the surface of the image material by any convenient means such as by a brush or stripper or by blowing it off with compressed air. A different image may then be produced on the image carrier material and this image transferred to image-receiving material is described hereinabove.

For example, a commercially available conventional elcctrophotographic material consisting of an aluminum base and a photoconductive image layer was charged in the conventional manner and exposed to the mirror image of an original document. The resultant latent electrostatic image was developed with a finely divided iron powder and the developed image fixed in place by means of a magnetic field applied from beneath the supporting aluminum base.

The magnetically fixed image thus produced was used for the purpose of producing electrostatic images. A plastic coated paper, the back of which was grounded, was contacted with the iron powder image formed in relief on the image plate and a charge was imposed through the aluminum base. The imposed charge varied in a series of tests from 3 kilovolts to kilovolts. The latent charge image obtained on the paper was developed by means of conventional triboelectric toners and fixed by conventional means.

Although electric charges of between about two (2) to twenty (20) kilovolts have been employed, it is preferred to employ an electric charge on the magnetic, electrically conductive toner of between about three (3) to about ten (10) kilovolts and most preferably between about six (6) to about eight (8) kilovolts.

The invention has been described in detail with reference to some preferred embodiments thereof. Modifications and alterations will occur to others upon reading and understanding the specification, which modifications and alterations fall within the spirit and scope of this invention. It is intended to include all such modifications and alterations within the scope of the appended claims or the equivalents thereof.

What is claimed is: 1. An electromagnetographic process comprising generating a latent electrostatic image on an image carrier consisting essentially of a photoconductive layer supported on an electrically conductive, magnetically neutral base, developing said image with a conductive toner comprising electrically conductive, magnetizable particles.

holding the conductive toner particles in place on the image carrier by means of a magnetic field,

imposing a transferable electrical charge on the conductive toner particles, and contacting said particles with an insulative image-receiving material supported on an electrically conductive base, thereby inducing an electrical charge image from said particles onto said image-receiving material.

2. The process of claim 1 wherein said image carrier is substantially in the form of a cylinder.

3. The process of claim 1 wherein said image receiving material is in the form of a roll of material.

4. The process of claim 3 wherein said image receiving material is a plastic coated paper.

5. The process of claim 1 wherein said image carrier is substantially in the form of a cylinder and said image receiving material is in the form of a roll of plastic coated paper.

6. The process of claim 1 further including repeating the steps of imposing a transferable charge on the toner particles and contacting said particles with the image-receiving material whereby multiple electrostatic images are induced upon said image-receiving material.

7. The process of claim 1 further including developing and fixing the images induced on said image-receiving material.

8. The process of claim 1 further including extinguishing said magnetic field and removing said toner particles from said image carrier after the production of said images on said image-receiving material.

9. The process of claim 1 wherein said conductive toner comprises particles of a material selected from the group consisting of iron, cobalt, nickel, alloys thereof, and mixtures of the foregoing.

10. The process of claim 1 wherein said conductive toner comprises particles of an electrically conductive, magnetizable metal.

11. The process of claim 1 wherein said conductive toner comprises a plastic carrier containing one or more electrically conductive, magnetizable materials.

12. The process of claim 1 wherein the charge imposed upon said conductive toner particles is between about 2 to about 20 kilovolts.

13. The process of claim 12 wherein said charge is between about 3 and about 10 kilovolts.

14. The process of claim 13 wherein said charge is between about 6 and about 8 kilovolts.

15. The process of claim 1 wherein said image-receiving material has an electrical conductivity of about 10- mhos per centimeter.

16. An electromagnetographic process comprising generating a latent electrostatic image on an image carrier consisting essentially of a photoconductive layer sup ported on an electrically conductive, magnetically neutral ase,

developing said image by disposing a conductive toner comprising electrically conductive, magnetizable particles over said image, maintaining said conductive toner particles in place by means of a magnetic field, contacting said particles with an insulative image receiving material supported on an electrically conductive base,

imposing a transferable electric charge on said conductive toner particles while said particles are in contact with said image-receiving material, thereby inducing an electrical charge from said particles to said image-receiving material, repeating the foregoing steps at least once to produce at least two electrostatic images on said image-receiving material and developing and fixing the electrostatic images so produced.

17. The process of claim 16 wherein said latent electrostatic image is generated on a photoconductive sur face by exposing a uniformly charged photoconductive surface to an object or image thereof.

18. The process of claim 15 wherein said image carrier is substantially cylindrical in form.

19. The process of claim 11 wherein said image-receiving material is dispensed from a roll of said material.

20. The process of claim 16 further including extinguishing said magnetic field and removing said toner particles from said image carrier after the production of said images on said image-receiving material.

21. An electromagnetographic process comprising generating a latent magnetic image on an image carrier 7 supported on an electrically conductive, magnetically Ref rences Cited fii ii cffif isifi iii iiciii $333635? li f z iiii UNITED STATES PATENTS particles p g y 3,250,636 5/1966 Wilferth 117-17.s holding the conductive toner particles comprising said 5 3,485,621 12/1969 Kazan 96 1 R image in Pm by means ijisifiii 1%???) E3336? "15.212222? 36 31? an imposed magnetic field,

imposing a transferable electrical charge on the conductive toner particles, and contacting said particles GEORGE LESMES Pnmary Exammer with an insulative image-receiving material sllp- 10 R. E. MARTIN, JR., Assistant Examiner ported on an electrically conductive base, thereby inducing an electrical charge image from said par- US. Cl. X.R. ticles into said image-receiving material. 1l717.5 

